The pregnant uterus is an immunologically privileged site. Despite major histocompatibility complex (MHC) and minor H differences, rejection of the fetus by mother is rare and acceptance of migrant maternal cells by the offspring is common. We recently showed a strong neonatal tolerance effect of maternal antigen exposure in mice. This effect, which required both gestation and lactation-phase exposure, resulted in acceptance of maternal antigen + heart allograft for >100 days in 40-50% of recipients, without chronic rejection. The goal of the proposed work will be to test the hypothesis that maternal microchimerism arising by transplacental migration of maternal stem cells and sustained by oral exposure to maternal antigens in the neonate, induces allotolerance while reinforcing self antigen-specific tolerance. This hypothesis will be tested by means of three specific aims: 1) We will determine the influence of maternal exposure upon the development and phenotype of maternal antigen specific T regulatory and T effector cells in an F1 backcross breeding model that results in transplant tolerance to maternal antigens;2) We will examine the strain differences in mouse F1 back-cross breeding models that exhibit either tolerance or rejection of heart allografts carrying the non-inherited maternal antigens;in particular we will analyze the role of maternal hematopoeitic microchimerism in sustaining neonatally induced tolerance or sensitization in the adult, the extent of parenchyma! cell microchimerism, and the role of intercellular membrane transfer associated with dendritic cells of the offspring;and 3) We will investigate the peculiar resistance of the long-term surviving, maternal antigen + heart allograft to chronic rejection by testing the hypothesis that maternal microchimerism induces T regulatory cells that can suppress autoimmunity to cardiac myosin in susceptible mouse strains. SUMMARY: We believe that if successful, this proposal will advance our understanding of the mechanism(s) of neonatal tolerance to maternal antigens, an area of great significance for the health of mother and baby, especially in autoimmune disease-susceptible individuals. It will also provide fundamental new insights in the field of allo-tolerance, the "holy grail" of transplant immunology.